The question is far too broad. The answers could fill a website all on their own. But, there are books that touch on this, primarily these two:
Note that even these books are not a complete guide, but are instead a collection of ideas and approaches you can use, some of which won't work together, or are even contradictory. Typically it assumes some experience developing games, network applications, or ideally both.
(Note that I'm talking more about the MMOs in the original question - a 'network game' could mean all sorts of things from a PHP-based text game right through to an MMO, and the sub-questions above don't all apply to each type.)